Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy. In one type of external beam radiation therapy, an external radiation source is used to direct a sequence of X-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
The term “radiotherapy” refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. The amount of radiation utilized in radiotherapy treatment sessions is typically about an order of magnitude smaller, as compared to the amount used in a radiosurgery session. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centiGray (cGy)), short treatment times (e.g., 10 to 30 minutes per treatment) and hyperfractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted.
One challenge facing the delivery of radiation to treat pathological anatomies is identifying the target region at a particular point in time because the pathological anatomies may move as a function of the patient's breathing or other natural movements. In radiation treatment, it is useful to accurately locate and track the motion of a target region due to respiratory or other patient motions during the treatment. In order to perform radiation treatment in organs near, for example, the abdomen, lungs, liver, or pancreas, it is useful to take into account the movement of these structures during the patient's respiratory cycle. Conventional methods and systems have been developed for tracking of an internal target region, while measuring and/or compensating for breathing and/or other motions of the patient.
In one conventional method, instead of prescribing a dose solely to the target region, a margin around the target region is defined so that the entire volume traversed by the target region during free breathing receives the prescription dose. Another conventional method controls the amplitude of the patient's respiration, for example, by using a restraint on the chest, so that tissue movement is reduced. A treatment margin is defined, but in this case a smaller treatment volume is used to reflect the reduced amplitude of motion.
Other conventional methods utilize breath holding and respiratory gating to compensate for target region movement during respiration while a patient is receiving conventional radiation treatments. Breath holding is implemented by a patient holding his or her breath at the same point in each breathing cycle, during which time the tumor is treated while it is presumably stationary. A respirometer is often used to measure the tidal volume—the inhaled volume or the change in lung volume during inhalation—and ensure the breath is being held at the same location in the breathing cycle during each irradiation moment. This method takes a relatively long time and often requires training the patient to hold his or her breath in a repeatable manner.
Respiratory gating involves a process of measuring the patient's respiratory cycle during treatment and then turning the radiation beam on only for a predetermined part of the patient's breathing cycle. Respiratory gating does not directly compensate for motions that result from breathing. Rather, radiation treatment is synchronized to the patient's breathing pattern, limiting the radiation beam delivery to times when the tumor is presumably in a reference position. The time taken to treat a patient with respiratory gating is related to the width of the “window” in the breathing cycle during which the beam is enabled. Hence, there is a compromise needed between a wide window (short treatment time, but large amount of target motion during treatment) and a narrow window (small target motion, but long treatment time). Respiratory gating methods also may require the patient to have many sessions of training over several days to breathe in the same manner for long periods of time. Conventional respiratory gating also may expose healthy tissue to radiation before or after the tumor passes into the predetermined position. This can add an additional margin of error of, for example, about 5-10 millimeters (mm) on top of other margins normally used during treatment. However, the prescription volume can usually be smaller than that using free breathing without gating. These conventional methods are limited by the patient's ability to perform breathing functions in a consistent manner over multiple treatment sessions.
Another conventional method of dealing with the motion of a target region during radiation treatment involves the image tracking of fiducial markers that are placed in or near the target region. The position and motion of the fiducial markers is correlated with the position and motion of the target region so that real-time correction of the position of the treatment beam to follow the motion of the target region may be realized, using a real-time continuous imaging method (e.g., fluoroscopy) to continually track the position of the fiducial markers.
Another method of tracking target motion during radiation treatment involves implantation of fiducial markers in or near the target region, as well as the use of non-invasive devices that may be tracked in real time. For example, light emitting diodes (LEDs) may be attached to the skin of the patient's chest and tracked by a camera in the treatment room. The fiducial markers are imaged intermittently, e.g. using X-ray imaging in the treatment room, and a correlation model is built between the positions of the fiducial markers and the positions of the LEDs. Using the real time information on the LED positions, the position of the target is estimated using the correlation model, and the position of the treatment beam is updated accordingly.
Each of these techniques has certain advantages and drawbacks. Without restraint or gating, a fast treatment is possible that is comfortable for the patient. However, some approaches result in the irradiation of a volume of tissue substantially larger than the target region, especially in regions where respiratory motion is large, such as near the diaphragm. Controlling respiratory amplitude can make treatment uncomfortable, and gating causes an increase in treatment time. Performing real-time correction according to the movement of fiducial markers implanted in the target region allows a conformal dose distribution to be delivered quickly. Nevertheless, this method does have a disadvantage that it requires invasive fiducial implantation, and in the case that continuous X-ray imaging is used during treatment, the imaging component itself delivers a substantial dose of radiation to healthy tissue. Real-time correction according to the movement of fiducial markers also may require a radiation delivery device that can be moved quickly and accurately. One such radiation treatment system is the CYBERKNIFE® system developed by Accuray Incorporated, of Sunnyvale, Calif. By mounting a compact X-band linear accelerator on a robot arm assembly, the CYBERKNIFE® radiation treatment system can perform real-time compensation for respiratory motion.
One conventional treatment planning approach using a CYBERKNIFE® radiation treatment system utilizing inverse planning techniques is as follows. First, a target region to be treated and critical structures to be avoided are delineated on a CT scan, or a set of CT slices of a section of the patient's anatomy. More specifically, a three-dimensional (3D) CT scan is composed of a three-dimensional model of section of the patient (e.g., pathological anatomy bearing portion of the body) generated from a collection of two-dimensional (2D) CT slices, with each slice representing a different position in space (for example, a different position along the inferior-superior axis of the patient). In CT scanning, numerous X-ray beams are passed through a section of the body at different angles. Then, sensors measure the amount of radiation absorbed by different tissues. As a patient lies on a couch, an imaging system records X-ray beams from multiple points. A computer program is used to measure the differences in X-ray absorption to form cross-sectional images, or “slices” of the head and brain. These slices are also called tomograms.
Once the target region and critical structures have been delineated, dose constraints may then be applied by a medical physicist to these target regions and critical structures. The medical physicist specifies the minimum dose, and optionally the maximum dose, to the tumor and the maximum dose to other healthy tissues independently. The treatment planning software then selects a set of treatment beam parameters (e.g., direction, total number of beams and duration of each beam) in order to achieve the specified dose constraints. Next, the dose constraints may be altered, tuning structures may be added, and the treatment plan re-optimized until the dose distribution is acceptable. The finalized treatment plan is then sent to a treatment delivery system.
Some conventional treatment planning and delivery systems also implement spatial smoothing functions to represent the deformation of the patient's anatomy during respiration. Spatial smoothing is based on principles of spatial continuity, which is the understanding that adjacent physical points of an object are joined in a continuous manner. As an example, a metal bar exhibits the characteristics of spatial continuity. The many points along a metal bar remain adjacent to one another in a continuous manner as the bar is flexed or bent. In contrast, when the bar is broken, adjacent points move in a non-continuous, or discrete, manner so that they do not remain continuously adjacent to each other. Like a flexed metal bar, physical organs and pathological anatomies are assumed to be spatially continuous. Even though an organ or pathological anatomy may deform over time, the adjacent points of the organ or pathological anatomy are assumed to remain adjacent at all points in time, under normal conditions. Thus, the physical deformations of an organ or pathological anatomy typically conform to the assumptions of spatial continuity. Some conventional radiation treatment systems may implement spatial smoothing functions based on the assumptions of spatial continuity.
Temporal continuity, in contrast to spatial continuity, relates to the movement of a single point over time. In particular, temporal continuity is the understanding that a single point moves along a continuous path of motion over time. In other words, the point does not jump from one location to a non-adjacent location without passing along a continuous path between the two non-adjacent locations. Conventional radiation treatment systems do not use the concept of temporal continuity to model tissue deformation during respiration.